1. Field of the Invention
The invention relates to a method and an apparatus for leveling an increasing or decreasing slope of an AM modulated receiving signal.
2. Related Technology
In an amplitude modulated receiver—AM receiver—the level of an amplitude modulated receiving signal is controlled by means of an automatic gain control (AGC) for adapting the receiving signal to the predetermined signal range in the signal path.
The receiving signal is fed through a (pre-)amplifier 1 according to FIG. 1. The level of the (pre-) amplified receiving signal is measured by a subsequent RF level detector 2. The automatic gain control 3 determines the suitable attenuation value for the radio-frequency receiving signal. Adjusted by means of the automatic gain control 3 the radio-frequency-attenuator (RF attenuator) 4 attenuates the receiving signal for ensuring a suitable signal level at the broadband mixer 5 input.
The mixer 5 supplied with a carrier signal from a local oscillator 6 converts the receiving radio frequency signal to intermediate-frequency (IF) range. The intermediate-frequency signal is bandpass filtered for a subsequent narrowband analog-digital-converter 10 by means of an intermediate-frequency filter 7.
The level of the intermediate-frequency receiving signal is measured by a subsequent IF level detector 8. The automatic gain control 3 determines the suitable attenuation value for the intermediate-frequency receiving signal. Receiving the determined attenuation value from the automatic gain control 3 the intermediate-frequency-attenuator (IF attenuator) 9 attenuates the receiving signal for ensuring a suitable signal level at the (narrowband) analog-digital-converter 10 input.
After analog-digital-converting the attenuated digital receiving signal is frequency shifted to the baseband by means of a digital-down-converter 11 sourced with a carrier signal from a numerically controlled oscillator 12. Finally the baseband receiving signal is filtered and demodulated in an AM demodulator 13. In the AM demodulator 13 the time average DC(m(n)) of the amplitude m(n) of the complex attenuated baseband receiving signal x(n) is determined by lowpass filtering of the amplitude m(n) of the complex attenuated baseband receiving signal x(n) in a lowpass infinite-impulse-response filter 14.
In a subtracting unit 15 the difference between the amplitude m(n) of the complex attenuated baseband receiving signal x(n) and the time average DC(m(n)) of the amplitude m(n) of the complex attenuated baseband receiving signal x(n) is calculated, which is scaled with the time average DC(m(n)) of the amplitude m(n) of the complex attenuated baseband receiving signal x(n) in a subsequent scaler 16 for generating the AM demodulated receiving signal y(n) corresponding to equation (1).
                              y          ⁡                      (            n            )                          =                                            m              ⁡                              (                n                )                                      -                          D              ⁢                                                          ⁢                              C                ⁡                                  (                                      m                    ⁡                                          (                      n                      )                                                        )                                                                          D            ⁢                                                  ⁢                          C              ⁡                              (                                  m                  ⁡                                      (                    n                    )                                                  )                                                                        (        1        )            
For cost and performance reasons the RF and IF attenuator 4 and 8 are usually realized as step attenuators exhibiting only discrete levels of attenuation, for example 1-12 dB in 1 dB steps as in curve 1 of FIG. 2.
A RF receiving signal with an increasing and/or decreasing slope—curve 2 in FIG. 2—is transformed by such an attenuation to an attenuated RF receiving signal with smaller increasing and/or decreasing slope segments connected each other by step segments. Curve 3 in FIG. 2 shows the amplitude m(n) of the complex attenuated baseband receiving signal x(n) at the output of the digital-down-converter 10.
The subsequent AM demodulator creates the demodulated signal y(n) from the attenuated baseband receiving signal x(n). In principle the AM demodulation is performed corresponding to equation (1) by determining the time average DC(m(n)) of the amplitude m(n), subtracting the time average DC(m(n)) from the amplitude m(n) and normalizing the difference by dividing through the time average DC(m(n)) of the amplitude m(n).
The time average DC(m(n)) of the amplitude m(n) of the complex attenuated baseband receiving signal x(n) is determined by means of a lowpass infinite-impulse-response filter. The order of the lowpass infinite-impulse-response filter is normally at least two—typically four—and therefore the lowpass infinite-impulse-response filter, if activated by an step in the signal at its input, generates oscillations. The signal at the output of the lowpass infinite-impulse-response filter representing the time average DC(m(n)) of the amplitude m(n) of the complex attenuated baseband receiving signal x(n) is characterized by oscillations in case of steps in the amplitude m(n) of the complex attenuated baseband receiving signal x(n), as can be seen in curve 1 of FIG. 3.
These oscillations are transferred to the output signal y(n) of the AM demodulation (curve 2 of FIG. 3). In case of a audio signal as receiving signal the oscillations in the output signal y(n) can be heard as unwanted clicks.
A receiver with an AGC-controller and a predictive offset correction is known from U.S. Pat. No. 6,240,100 B1 referred to as background art.